The invention relates generally to the field of image processing, and in particular to methods for detecting when an image has been rotated or resized (magnified).
U.S. Pat. No. 5,835,639 issued Nov. 10, 1998 to Honsinger et al., entitled xe2x80x9cMethod For Detecting Rotation And Magnification In Imagesxe2x80x9d describes a method for embedding two identical carriers in an image in order to track rotation and magnification changes. The method is based on the fact that the autocorrelation of the image will contain the cross-correlation of the two carriers and the cross-correlation will exhibit two delta functions whose distance apart will be proportional to magnification and whose angle will be maintained under no rotation or will be rotated in the same amount as the rotation of the original image. Thus, by identifying the location (separation and rotation) of the two delta functions one can determine the amount of rotation and magnification that an image has undergone. In images that contain a large amount of pronounced linear and angular features, these features tend to cause the autocorrelation of the image to exhibit a number of features that mask the delta functions, thereby making the detection of the delta functions difficult.
There is a need therefore for an improved method of detecting angular rotation and magnification of the type disclosed in the above referenced patent that avoids this problem.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention an improved method and computer program product for detecting an amount of rotation or magnification in a modified image, of the type including the steps of:
a) embedding a marker image having a pair of identical features separated by a distance d and oriented at an angle xcex1 in an original image to produce a marked image, the marked image having been rotated and/or magnified to produce the modified image;
b) performing an autocorrelation on the modified image to produce a pair of autocorrelation peaks corresponding to the location of the features of the marker image in the modified image; and
c) comparing the separation d and orientation xcex1 of the autocorrelation peaks with the separation d and orientation xcex1 of the features in the marker image to determine the amount of rotation and magnification in the modified image, wherein the improvement comprises the step of:
d) preprocessing the modified image to have a constant standard deviation over all local regions in the image.
The present invention has the advantage of being able to more easily detect the autocorrelation peaks in an image having large amounts of linear and angular detail.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.